The present invention relates to a combined melt--solid state polymerization process to produce high molecular weight high purity polyesters. A maximized reaction rate for the solid state polymerization is achieved when the polyester prepolymer has an optimal carboxyl content.
Heretofore, polyesters have been produced by the conventional melt process and have failed to achieve a high molecular weight with minimal acetaldehyde concentrations. These polyesters so produced have generally failed to meet the stringent acetaldehyde content specifications for polyester products used in the production of beverage and food packaging. The acetaldehyde reduces the purity of the polyester product and even affects the contents within the packaging.
The production by a melt process has been the conventional production method for polyesters. For example, U.S. Pat. No. 3,551,386 discloses the process for the production of polyester utilizing the conventional melt process. However, because a conventional melt process has these inherent deficiencies for the production of polyesters for certain commercial and industrial applications, a method for the production of a polyester avoiding these deficiencies is needed.
There have been attempts to produce the polyester by a conventional process in combination with a solid state polymerization. However, the polyester prepolymer produced by the conventional melt process has a very low reaction rate during solid state polymerization, because the carboxyl content of the conventional prepolymer is generally less than 12% of total end group content. Therefore, there must be adjustments to the production of the polyester prepolymer which will maximize the reaction rate in the solid state polymerization by producing a polyester prepolymer with an optimal carboxyl content.